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In physics, a free surface flow is the surface of a fluid flowing that is subjected to both zero perpendicular normal stress and parallel shear stress.This can be the boundary between two homogeneous fluids, like water in an open container and the air in the Earth's atmosphere that form a boundary at the open face of the container.
In hydrodynamics, the free surface is defined mathematically by the free-surface condition, [11] that is, the material derivative on the pressure is zero: = In fluid dynamics , a free-surface vortex , also known as a potential vortex or whirlpool, forms in an irrotational flow, [ 12 ] for example when a bathtub is drained.
This includes pressure inlet and outlet conditions mainly. Typical examples that utilize this boundary condition include buoyancy driven flows, internal flows with multiple outlets, free surface flows and external flows around objects. [1] An example is flow outlet into atmosphere where pressure is atmospheric.
These conditions are used when we don’t know the exact details of flow distribution but boundary values of pressure are known For example: external flows around objects, internal flows with multiple outlets, buoyancy-driven flows, free surface flows, etc. The pressure corrections are taken zero at the nodes.
If the free surface elevation η(x,t) was a known function, this would be enough to solve the flow problem. However, the surface elevation is an extra unknown, for which an additional boundary condition is needed. This is provided by Bernoulli's equation for an unsteady potential flow. The pressure above the free surface is assumed to be constant.
In fluid dynamics, flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. [1] A boundary layer exists whenever there is relative movement between a fluid and a solid surface with viscous forces present in the layer of fluid close to the surface. The flow can be externally, around a body ...
These two publications provided more details about the specific procedures used to approximate the position of the free surface (locally represented by an inclined line in surface cells) and apply the free surface boundary conditions on it. Since VOF method surpassed MAC by lowering computer storage requirements, it quickly became popular.
The following applications involve the use of Neumann boundary conditions: In thermodynamics, a prescribed heat flux from a surface would serve as boundary condition.For example, a perfect insulator would have no flux while an electrical component may be dissipating at a known power.